1. Field of the Invention
The present invention relates to an automatic transmission, more particularly to a lock-up control system provided with a lock-up clutch in a torque converter.
2. Description of the Prior Art
In an automatic transmission having a torque converter and a transmission mechanism controlled in accordance with a shift pattern on a map to provide automatically one of a plurality of gear shift stages, it has been proposed to provide a lock-up clutch between an input member and output member of the torque converter for connecting the input member with the output member directly in accordance with an operating condition of a vehicle so as to improve an efficiency in transmitting an engine power. Assignee of the present application owns U.S. Pat. Nos. 4,580,671, 4,618,041, 4,669,441 and 4,687,083 which disclose conventional transmissions.
It should however be noted that the when a gear shift operation occurs in the transmission in accordance with the map in the case where the lock-up clutch is engaged, a torque shock between an engine side and a wheel side of the vehicle body caused by the shift operation is directly transmitted to the vehicle bypassing the torque converter resulting in a deterioration of a riding comfort. In view of this problem, there has been known a control for the automatic transmission in which the lock-up clutch is released to transmit the torque from the engine to the transmission through the torque converter until the shift operation is completed in the transmission.
However, even when the lock-up clutch is released to ease the torque shock caused by the automatic shift operation, the torque converter exerts an inherent torque amplification so that the torque shock cannot be removed effectively. In addition, when the lock-up clutch is engaged after the completion of the shift operation, an torque shock might be produced undesirably.
Under the circumstances, Japanese Patent Public Disclosure No. 57-33253 filed on Aug. 4, 1980 and laid open to the public in 1982, corresponding to U.S. Pat. No. 4,468,988 discloses a lock-up control system for an automatic transmission with a lock-up clutch in which under a specific driving condition of the vehicle, the lock-up clutch is maintained at a half-engaged condition allowing a certain slippage of the lock-up clutch in a manner that a rotation speed difference between the input shaft and the output shaft of the torque converter is controlled to a predetermined value by means of a feedback control. However, the slip control in the lock-up control system as disclosed in the Japanese Patent Public Disclosure No. 57-33253 is not so effective to ease the torque shock in a shift operation. This is because the slip control is not carried out when the shift operation occurs in a driving condition other than the specific driving condition.